1. Field of the Invention
The present invention relates to an internal combustion engine with a dual turbocharger system having a first turbocharger and a second turbocharger in which air is charged to the engine by only the first turbocharger at small to medium intake air quantities and is charged by both the first and the second turbochargers at large intake air quantities.
2. Description of the Prior Art
As illustrated in FIG. 6 (Prior Art), a conventional turbocharged internal combustion engine 1' is provided with a first turbocharger 7' and a second turbocharger 8' having turbines 7a' and 8a', respectively, and compressors 7b' and 8b', respectively. An exhaust switching valve 17' is installed in an exhaust conduit 20b' positioned downstream of the turbine 8a'. Similarly, an intake switching valve 18' is installed in an intake conduit 14b' positioned downstream of the compressor 8b'. Further, an intake bypass valve 33' is installed in an intake bypass conduit 13' bypassing the compressor 8b'. The intake switching valve 18' and the exhaust switching valve 17' are closed at small to medium intake air quantities to produce "one-turbocharger-operation" wherein only the first turbocharger 7' is operated. The intake switching valve 18' and the exhaust switching valve 17' are opened at large intake air quantities to produce "two-turbocharger-operation" wherein both the first turbocharger 7' and the second turbocharger 8' are operated. A smooth transition from a "one-turbocharger-operation" to a "two-turbocharger-operation" is obtained by partially opening the exhaust switching valve 17' to run-up the second turbocharger 7'.
Japanese Patent Publication SHO 59-145328 teaches partially opening the exhaust switching valve 17' at an intake air quantity smaller than a switching intake air quantity at which a "one-turbocharger-operation" is changed to "two-turbocharger-operation". Also, Japanese Patent Publication SHO 61-112734 teaches partially opening the exhaust switching valve 17' at a charging pressure lower than a switching charging pressure at which a "one-turbocharger-operation" is changed to a "two-turbocharger-operation".
However, partially opening the exhaust switching valve as described above can degrade system performance. More particularly, in the case where the partial opening of the exhaust switching valve 17' is controlled on the basis of an intake air quantity, the charging pressure will not reach the desired charging pressure due to a time lag in turbocharger operation and therefore, the transition from a "one-turbocharger-operation" to a "two-turbocharger-operation" is accompanied by a decrease in torque at low engine speeds. In contrast, in the case where partial opening of the exhaust switching valve 17' is controlled on the basis of a charging pressure, the turbocharger operation is changed from a "one-turbocharger-operation" to a "two-turbocharger-operation" without experiencing the step of partially opening the exhaust switching valve 17' and therefore, the transition is accompanied by a relatively large torque shock.